1. Field of the Invention
Embodiments of the invention generally relate to a method and apparatus for subliming a solid to provide a gas to a micromachining processing chamber.
2. Description of the Related Art
Semiconductor processing and micromachining use a wide variety of process gases. As structures and devices become increasingly complex, it would be advantageous to provide greater variety of process chemistries. However, some processes gases may be prohibitively expensive to refine, difficult to handle or suffer from other shortcomings such as highly reliable gas delivery techniques.
Process gases used during semiconductor processing and micromachining are typically provided to the gas panel in liquid or gaseous form from a central source or a supply vessel positioned proximate the gas panel. Some process gases may be generated at or near the gas panel from solid material through a sublimation process. Sublimation is generally the process through which a gas is produced directly from a solid at a certain pressure and temperature without passing through a liquid state. Some gases that may be produced through a sublimation process include xenon difluoride and nickel carbonyl, among others. As these materials tend to be very reactive and expensive, careful control of the sublimation process is required in order to manage the generation of sublimed process gases without undue waste.
A conventional sublimation process is typically performed in a heated vessel loaded or filled with the solid material to be sublimed. As gas is needed, the vessel walls and or tray supporting the solid material are heated and gas is produced. However, this method has a number of drawbacks.
Primarily, it is difficult to control heat transfer through the walls of the vessel. This results in inefficient consumption of the sublimed solids. The sublimation reaction driven by the heated walls of the vessel consumes the outer portions of the solids contained in the vessel. As many sublimed gases have a propensity to coagulate with the generating solid upon cooling, the solid coagulates at the center of the vessel, substantially reducing the surface area available for future sublimation.
Additionally, the temperature gradient within the vessel results in difficulty controlling the volume of sublimed process gas produced. Once a desired amount of process gas has been produced, residual heat of the vessel walls continues to undesirably drive the sublimation reaction, thereby producing an excess of process gas. The production of more gas than necessary drives up process costs and additionally requires frequent process interruption to recharge the crystals within the vessel. The residual gas also may attack the components within the gas delivery system.
Moreover, some sublimed gases, such as xenon difluoride, have a propensity to deposit on passages of the vessel and subliming crystals. Thus, prevention of excess process gas generation/formation prevents clogging of vessel passages. Additionally, preventing the subliming crystals from coagulating with re-deposited material maintains the surface area available for future sublimation, thus improving the gas generation uniformity over a larger process window.
Therefore, a need exists for an improved method and apparatus for providing sublimed gases to a processing chamber.